Recast part 3: wrapper classes

Adds NavMesh and NavPath classes that use Recast and Detour to auto-generate and pathfind on navmeshes. Also adds icons to the world editor creator menu. Basically a stripped-down version of the Recast Resource, with some architectural improvements from Walkabout.

The code is activated by including the 'navigation' module in the project generator - again, thanks to Luis Rebollo for figuring that part out.

I've decided to submit three pull requests for recast since there are three distinct sets of changes to be made.
 1. #431 Adding Recast library code
 2. #432 Making changes to the Torque engine to accommodate getting polys for Recast
 3. This request, actual classes that wrap and make use of Recast/Detour
I figured it would be easier to break these up - especially number 2, since people will probably have differing opinions on my changes.